Sensible and Latent Heat

Energy interacts with substance in two distinct ways. The key difference is that it changing temperature or phase.

1. Sensible Heat (Temperature change)

Sensible heat is energy that changes the temperature (dry-bulb temperature) of a substance without changing its state (phase). It is “sensible” because you can sense it and measure it with a thermometer.

• Example: Heating water from 25°C to 89°C. The water stays liquid, but it gets hotter.
• HVAC related: This is how we control air temperature by adding or removing sensible heat.

2. Latent Heat (Phase change)

Latent heat is energy that change the phase of a substance (e.g., solid to liquid or liquid to vapor) while the temperature stays exactly the same. The energy goes into breaking molecular bonds rather than increasing heat.

• Example: Boiling water. The water stays at 100°C even as it turns into steam, despite constant heat being added.
• HVAC related: This is the “hidden” load used for humidity control (adding or removing water vapor from the air).

Examples

In HVAC design, we must calculate both loads to keep a space comfortable or at indoor design condition. Here are three example scenarios:

1. The Hot-Pot (Shabu-shabu) Restaurant
   • Sensible: The heat radiating from the electric stove and the hot pot itself warms up the air in the room.
   • Latent: As the soup boils, it releases a massive amount of steam (water vapor). The air conditioner must work to “dehumidify” this moisture so the restaurant doesn’t feel like a sauna.
2. People in an Office
   • Sensible: Our bodies naturally radiate heat (like a small heater), which raises the room temperature.
   • Latent: We also breathe out moisture and sweat. This adds water vapor to the air, increasing the humidity load.
     
3. Computers (PCs) in an Office
   • Sensible: Electronics generate heat through electricity. This is 100% sensible heat because computers don’t breathe or sweat. They only make the room hotter, not more humid.
   • Latent: Zero.

Summary: Sensible heat changes how hot it feels; latent heat changes the moisture level in the air.

ความร้อนสัมผัส (Sensible heat) คือ ความร้อนที่ทำให้สสารมีอุณหภูมิ (Dry-bulb temperature) เปลี่ยนแปลงโดยไม่มีการเปลี่ยนสถานะ (อุณภูมิเปลี่ยนแต่สถานะคงเดิม)

ความร้อนแฝง (Latent heat) คือ ความร้อนที่ทำให้สสารมีการเปลี่ยนสถานะโดยที่อุณภูมิไม่เปลี่ยนแปลง (เปลี่ยนสถานะแต่อุณหภูมิคงที่)